With increased competitiveness in the aircraft industry, driven by considerations of size, weight, fuel efficiency, and jet noise, there is an interest in aircraft sound insulators which are lightweight and capable of serving as an effective sound attenuator for jet and high-speed air noises.
The need to reduce jet engine noise is particularly important, both from the point of view of limiting take-off noise at airports, and shielding passengers from engine noise. Noise restrictions in some urban airports limit the types of aircraft that may use the airport, or limit planes to below-maximum passenger and baggage loads.
Currently, engine noise on jet aircraft is damped primarily by annular-ring, honeycomb-type damping structures disposed in the engine housing along intake and exhaust regions of the engine. Typically a stacked array of such structures, usually up to six structures, are employed, with each structure being designed to dissipate sound noise of a characteristic peak frequency of the engine. The peak frequencies are primarily in the 500-6,000 Hz range for both turbofan and turboprop engines.
The stacks of honeycomb structures add substantial weight and size to the engine housing, reducing aerodynamic efficiency. In addition, since the damping structures are designed to dissipate sound near a single peak frequency, non-peak frequency noise may be only poorly attenuated.
There is thus a need for jet engine acoustical insulation structure that is lightweight, compact, able to absorb sound frequencies over a broad frequency range, particularly in the 500 to 6,000 Hz region for turbofan and turboprop engines, and at the same time, able to withstand high operational temperatures of a jet engine.